(1) Field of the Invention:
The present invention relates to a hot rolled steel sheet adapted for ultra-deep drawing, and more particularly relates to a hot rolled steel sheet having high resistances against secondary-work embrittlement and brazing embrittlement and adapted for ultra-deep drawing, and a method for producing the hot rolled steel sheet through a hot rolling step which can surely and stably form a ferrite texture effective for improving these properties.
(2) Related Art Statement:
A hot rolled steel sheet to be used in a part, such as a compressor cover for air-conditioning apparatus or the like, which is demanded to have an ultra-deep drawability, must be high in resistance against embrittlement under an impact load at low temperature after the hot rolled steel sheet has been subjected to a primary working, such as drawing or like, that is, must be high in resistance against secondary-work embrittlement. Moreover, when the hot rolled steel sheet, after having been subjected to a primary working or subjected to a secondary working following the primary working, is subjected to a brazing treatment, the primarily or secondarily worked steel sheet is required not to crack due to the brazing embrittlement, that is, required to have a high resistance against brazing embrittlement.
There have hitherto been known, as a hot rolled steel sheet for deep drawing, a hot rolled steel sheet produced from an A killed steel or rimmed steel, each having a low carbon content (C: 0.02-0.07% by weight; hereinafter, "% by weight" is represented by merely "%"), through a hot rolling following by a coiling at high temperature, and a soft hot rolled steel sheet produced from a steel having an ultra-low C content (C: &lt;0.01%) and containing B or Nb, which is added to the steel in order to make soft the resulting hot rolled steel sheet. Recently, Japanese Pat. Application Publication No. 60-7,690 has disclosed a hot rolled steel sheet, which is produced from a low carbon rimmed steel having a C content of not higher than 0.10% and having an available Mn content limited to at least 0.10%, said available Mn content being a remainder after consumed in the form of oxide and sulfide, by subjecting a slab of the steel to a particular treatment of a combination of a low temperature heating (1,050.degree.-1,200.degree. C.) and a low temperature rolling (700.degree.-800.degree. C.).
In general, in the hot rolled steel sheet, the development of {111} recrystallization texture, which is effective for deep drawability, is difficult contrary to that in the cold rolled steel sheet, and the r value of a measure of deep drawability is about 1.0 at the highest (in the cold rolled steel sheet, the r value is generally about 1.3-2.2). However, the hot rolled steel sheet has a large thickness, and hence the sheet can be drawn more advantageously due to its large thickness inspite of its low r value than the cold rolled steel sheet.
Therefore, in the hot rolled steel sheet, it is rather important that the sheet has a low .DELTA.r value of the planar anisotropy relating to r value, and further the ductility of the sheet is more important than the low .DELTA.r value. That is, in the hot rolled steel sheet, the low r value can be compensated by the excellent ductility.
It is known that the hot rolled steel sheet embrittles noticeably after workings, such as drawing and the like, which are accompanied with shrinkage or flange deformation, and therefore it is an important requirement for the hot rolled steel sheet not to crack by the impact load after the primary working, that is, to be high in the resistance against the secondary-work embrittlement.
The hot rolled steel sheet is often used as a material for a vessel. In this case, the hot rolled steel sheet is subjected to various workings after the deep drawing. Brazing is one of such workings, which have a serious influence upon the property of the steel sheet.
Brazing is a simple method and is used fairly widely due to its excellent airtightness. However, when the hot rolled steel sheet is brazed under a state that a high residual tensile load still remains in the sheet, the sheet has a risk of being cracked due to the "brazing embrittlement". Therefore, the hot rolled steel sheet is often subjected to a stress relief annealing before the brazing of the sheet. However, it results in an increase of treating steps to carry out the stress relief annealing before the brazing, and hence such procedure is not preferable.
Accordingly, it is an important property demanded of the hot rolled steel sheet that the sheet has such a high resistance against brazing embrittlement that the sheet can be easily subjected to a secondary working or a brazing working without carrying out the stress relief annealing after the deep drawing.
The properties demanded to the hot rolled steel sheet for ultra-deep drawing are as follows.
(1) The sheet has a high ductility.
(2) The sheet has a low stress at the yield point.
(3) It is desirable that the sheet has a high tensile strength while keeping its ductility.
(4) The sheet is free from cracks during the drawing or cracks due to the impact after the drawing, that is, the sheet is high in the resistance against secondary-work embrittlement.
(5) The sheet is free from cracks in the secondary working, welding, brazing and the like carried out after the drawing, that is, the sheet is free from the deterioration of the resistance against secondary-work embrittlement and has high resistance against brazing embrittlement.
The object of the present invention is to provide a hot rolled steel sheet having all of the above described properties, and a method for producing the steel sheet.
The inventors have investigated with respect to the composition of a steel which can produce a hot rolled steel sheet having the above described various properties, and found out the following facts. In order to solve the requirements of the above described items (1) and (2), an ultra-low C steel containing Ti is used. Moreover, in order to solve the requirement of item (2), S is contained in the ultra-low C steel in an amount lower than the ordinary level (S=0.005-0.015%), and further the content of Ti is limited depending upon the amounts of C, N and S. In order to solve the requirement of item (3), B is contained in a steel, and the coiling temperature following the hot rolling of the B-containing steel is set to a low temperature. In order to solve the requirements of items (4) and (5), it is effective to use a steel having low contents of S and P and containing B, and to form fine ferrite particles by the selection of proper hot rolling condition. Based on the above described discoveries, the inventors have accomplished the present invention.